GOP lawmakers warn that imposing tariffs may result in stiff retaliation

WASHINGTON—Republican lawmakers are showing increasing resistance to President Donald Trump’s trade agenda, worried that his plans could hurt exports from their states and undermine longstanding U.S. alliances.

The concerns indicate that the biggest threat to Mr. Trump’s trade policy—which emphasizes new bilateral deals and a tougher stance against countries blamed for violating trade rules—is coming from his own party. The opposition from Republicans, who control both chambers of Congress, stands to complicate Mr. Trump’s...